The Engineer’s Guide to Decoding & Encoding
This book is concerned with advanced encoding and decoding of composite video. Composite video was originally designed as monochrome compatible system for broadcasting in which subcarrier based colour information was added to an existing line standard in a way which allowed existing sets to display a monochrome picture. A further criterion was that the addition of colour should not increase the bandwidth of the TV channel. In that respect composite video has to be viewed as an early form of compression. Although designed for transmission, the baseband composite signals could be recorded on videotape. In the case of NTSC and PAL, vision mixing was also possible on composite signals. As a result early colour studios were entirely composite. There was one coder at the camera control unit and one decoder at the viewer’s TV set.
Since the introduction of colour, new processes such as slow motion, standards conversion, DVEs, graphics and so on have come into being. These have in common the fact that they cannot operate upon composite signals. All processes which manipulate the image spatially will render meaningless any subcarrier based colour information. In a composite environment such devices need an input decoder and an output encoder and clearly these need to be of high quality. Television is currently in a state of change with many new transmission formats proposed. Some of these work in components, but if such formats are adopted it will be some time before composite transmission ceases. Other proposals seek to increase the performance of composite signals. In both cases a requirement for quality coding and decoding is clear. Even if a utopian component world came about tomorrow, decoding would still be necessary to view the enormous composite archives which have built up. Whilst the techniques vary, all composite signals have in common the need to include a subcarrier based chroma signal within the luminance band in such a way that it will be substantially invisible on an unmodified monochrome TV set. This is achieved in much the same way in all three systems.
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